To stabilize and direct his craft, a helicopter pilot manually actuates piloting means (control stick, attitude control system and pedals) to act on the piloting axes of the helicopter (main rotor or tail rotor). A lateral or longitudinal displacement of the control stick can be used to respectively adjust the lateral or longitudinal axis of the helicopter by modifying the incidence of the blades of the main rotor. The attitude control system can be used to adapt the engine power to the flight conditions by modifying the angle of attack of the blades of the main rotor and can be used to modify the rate of climb and/or longitudinal speed of the helicopter. The pedals can be used to orient the nose of the helicopter by modifying the angle of attack of the blades of the tail rotor. The movement of the piloting means is sent, by means of mechanical transmissions, assisted power-wise by hydraulic circuits, to the piloting axes. The mechanical transmissions are, more often than not, arranged in series one after the other forming mechanical transmission subsystems. The assembly consisting of a piloting means, the mechanical transmission and the associated hydraulic circuit constitutes a flight control linked to the piloting axis concerned.
Helicopters are often equipped with an automatic pilot system, comprising a computation means and actuators, which acts on the flight controls, under the control of the pilot, in order to carry out two main functions: a first function to assist the pilot, and a second automatic pilot function.
When it assists the pilot in the manual control of his helicopter, the automatic pilot system can be used on the one hand to damp the changes to the machine to facilitate its control by the pilot, and on the other hand to maintain the current flight configuration (lateral and longitudinal attitudes, and bearing) so enabling the pilot to temporarily let go of the piloting means without being placed in a flight configuration that would be dangerous.
When it is in automatic pilot mode, the automatic pilot system can be used to servo-control one or more flight parameters (altitude, vertical rate of climb, longitudinal speed, lateral speed, bearing, navigation, etc.) on one or more set-point values previously chosen by the pilot.
To act on a flight control, the automatic pilot system uses a “series actuator”, which is a linear mechanical actuator placed in series in the mechanical transmission subsystem. This actuator has a body and an output shaft, it is normally of the worm screw/nut type and it has a reduced power and a short response time. It converts an electrical control into a displacement of its output shaft relative to its body. The “series actuators” are said to be “mechanically irreversible”, namely that they are distorted only when an electrical control is applied to them. In particular when the automatic pilot system is out of operation, the “series actuators” have no effect on the control of the helicopter. A neutral position of the series actuator corresponds to the position where the free end of its output shaft is at mid-travel.
A distinction is made between two types of failures, or operating errors that can affect the servo-control of an actuator acting on a flight control and being able to disturb the displacement of the actuator:                a first type of failure covers all the failures originated from within the automatic pilot system, this type of failure relates, for example, to malfunctions of a series actuator or of a computer of the automatic pilot system producing the actuator position servo-control.        a second type of failure covers all the failures originated from outside the automatic pilot system; this type of failure relates, for example, to hydraulic circuit operating errors.        
The hydraulic circuit gives a power boost to the displacement, by the pilot or by the actuator, of the flight control and therefore assistance in piloting: with the hydraulic circuit active, the displacement of the flight control will require little energy from the pilot (in manual piloting mode) or from the actuator (in automatic pilot mode).
On a failure of the hydraulic circuit (a loss of hydraulic pressure, for example), the assistance is lost, and all the effort needed to displace the flight control will be supported fully either by the pilot (in manual piloting mode), or by the actuator (in automatic pilot mode). On the yaw flight control of the A109 helicopter for example, the pilot can overcome the loss of hydraulic assistance, involving a greater physical effort, but the actuator is not capable of this and therefore remains blocked in position.
In the prior art, on an automatic pilot system comprising a computer acting simultaneously on several flight controls corresponding to different piloting axes of the helicopter (roll, pitch and yaw), when a failure is detected, a servo-control monitoring device takes a safeguarding measure independently of the type of failure. This measure consists in completely disengaging the automatic pilot system, that is, disabling the generation of all the servo-controls intended for the actuators acting on flight controls, and recentring the position of the various actuators around their neutral position by means of independent power circuits.
This safeguarding measure is simple and very safe but it has the drawback of being very disadvantageous to the pilot of the helicopter. Indeed, the complete disengagement of the automatic pilot system leads to a significant increase in the pilot workload. To reduce the probability of complete disengagement of the automatic pilot system, it is necessary to limit the disabling of all the servo-controls produced by the computer to only those situations where the safeguarding measure cannot be reduced to disabling a single servo-control intended for a failed axis.
One solution of the prior art for solving this problem consists in using a dual hydraulic circuit, on all the flight controls of the helicopter, that is, on all the axes of the helicopter. When a failure occurs on a first hydraulic circuit, the second hydraulic circuit takes over and handles the function of the first hydraulic circuit. Only when both hydraulic circuits of all the axes of the helicopter fail simultaneously does the automatic pilot system no longer act.
This solution is not always implemented by the aircraft manufacturer given that it is not essential and it reduces economic viability. Such is the case on medium-sized helicopters, such as, for example, the A109LUH helicopter: for this craft, the loss of hydraulic assistance on the yaw axis is not prohibitive in as much as the pilot can still displace the flight controls and act on the tail rotor. However, on the main rotor, it is essential to have a hydraulic redundancy, in as much as the pilot cannot act on this rotor without hydraulic assistance.